SI and CI Engine

Table beside is describe comparison between Spark Ignition (SI) and Compression (CI) engine.

Otto cycle is more efficient at a given compression ratio and heat input.

CI engine has higher thermal efficiency due to higher of compression ratio so that CI engine is heavily built.

Compression ratio & Detonation

Compression ratio (r) = total volume / clearance volume

A higher compression ratio increase the pressure and temperature of the working mixture and decrease the concentration of residual gases and tendency of the engine to detonate.

Detonation.The efficiency of engine must be design for the highest compression ratio but maximum compression ratio can be used is limited by detonation.

In the auto-ignition theory it is assumed that the flame velocity is normal before the onset of auto-ignition whereas in detonation theory a true detonating wave formed by pre-flame reactions has been proposed as the mechanism for explosive auto-ignition.

Octane Number

Highest Useful Compression Ratio (HUCR) at which a fuel can be used without detonation in a specified test engine under specified operating condition and the ignition and mixture strength being adjusted to give best efficiency.

As HUCR value is only applicable to a particular engine and a given set of operating conditions another method has been devised to give an anti knock measure of the fuel. This method is determination of the octane number of the fuel.

In octane number test the performance of unknown gasoline is compared with that a series of reference fuels consisting of mixtures of iso-octane and n-heptane. Iso-octane is low boiling point branched chain compound and has a very slight tendency to knock and hence arbitrarily assigned an octane number of 100. n-heptane detonates rapidly and hence has been assigned an octane number of zero.